I've used garage band, logic, and had the pleasure to spend 12 hours over the course of 4 weeks with a pro tools audio studio.
Garage Band is really weak. I will not lie about that. Logic is definately for the professional musician. It allows for more controlled sound effects, neater editing, and automations for effects on each track. (I love messing with pitch bend automation, it sounds so awesome!!! : D)
Pro Tools Audio is WTF!?!?!?!?!?! It's messed up, but once you get used to it, it is one of the most fantastic music programs ever. For the extreme professional.
My personal favorite is Logic, then Pro Tools, then Garage Band. From this thread, I found Psycle, and now I can use my VST's!!!
BTW, is there another program that I can get for free that can use VST's? (I recently bought a 600 VST collection from an anonymous website.)